0:09Skip to 0 minutes and 9 secondsIn this lecture, we're going to consider the origins of life, and what we actually mean by life. Until recently, it was believed that life spontaneously arose. For example, flies would come out of rotting meat. But with the dawning of new ideas on evolution and life cycles, came ideas about where life came from. For example, Darwin suggested that new species arose by modification of the old. Pasteur said that life always springs from life. But with anything in science, once you start looking into subjects in more depth, more sub-questions come forward, and from these questions, from Darwin and Pasteur, you get ideas. Well, OK, if life came from life, where did life start? What was the origins of life?

0:59Skip to 0 minutes and 59 secondsAnd what do we mean by life? Now, the earth is about 4.6 billion years old. Life itself, originated about 4 billion years ago, but what do we mean? Again, it's using the word life. What do we mean by life? Anything that is considered living must be able to replicate itself, and it must be governed by the sphere and the process of natural selection. Anything this is living must have the following properties. It must be made up of units that can reproduce by division, the properties of which are inherited by the offspring. There must be random mutations and retention of the variation amongst the offspring.

1:45Skip to 1 minute and 45 secondsSome offspring must be more fit than others, and this will result in adaptation to the environment. So replication is the key to life. Without replication, anything that is itself cannot extend beyond itself. Therefore, in looking at the origin of life, one must look at the origin of replication. Now, there's a suggestion that replication began with self-replicating units, probably RNA molecules. Membranes then evolved surrounding the molecules, and these self-replicating RNA molecules became bacteria with about 1,000 genes. Replication, it is believed, then moved from RNA to DNA. DNA being more complex. This is the DNA molecule which carries all the genetic information on the chromosome. In bacteria, a chromosome will be approximately one millimetre long bundled all into a single cell.

2:47Skip to 2 minutes and 47 secondsSo the process of life involves a chromosome replicating itself, and the cell then dividing. So when we talk about replication being the key to life, another key point about life is that a fundamental property is the unit of life, the cell. So the cell-- all living organisms are based on the cell. The cell defines an inside and an outside environment, and it regulates what is inside. This regulation is an active process, and if it ceases to be regulating the cell, the cell dies. Life covers a huge size range. There are bacteria as small as half a micrometre to a whale as long as 10 metres, and yet, all that life is based on that one unit, the cell.

3:44Skip to 3 minutes and 44 secondsBut essentially, you've got the bacteria as a single cell, the whale is a multitude of cells all joined together. And this brings me to a key concept, that there is only one sort of life. There is one tree of life, where all life is related. It's worth noting that in the early life on earth, it only included bacteria, a sort of flat, ecological structure. No one ate each other. Each bacteria essentially could destroy each other through antibiotics and that sort of thing, but essentially, the structure was flat. There were no food chains, no food webs. During the first billion or so years on earth, there was little or no oxygen. Life respired anaerobically. Oxygen was a waste product.

4:42Skip to 4 minutes and 42 secondsIn fact, for organisms, it was toxic. And then you get the build up of photosynthetic organisms, generating oxygen, building up oxygen in the environment. And the process of aerobic respiration, the ability to respire using oxygen, evolved, and this is much more efficient than anaerobic respiration. And so cells were able to combine together to become multi-cellular animals. Organisms could eat each other. You've got the development of food chains. All because this process of aerobic respiration is so much more efficient than anaerobic respiration. So you get this builds up in time all based around oxygen. Oxygen also built up in the atmosphere, so you get the development of an ozone layer, which protects life from the harmful UV rays of the sun.

5:36Skip to 5 minutes and 36 secondsAnd this, in turn, allowed organisms to move onto land. And so oxygen is a fundamental process, this production of oxygen, the aerobic respiration, development of the ozone layer, was a fundamental process by which life was able to move from single cell to multi-cellular, and move onto land. So where we talk about life, we talk about the fundamental properties or replication, and the unit, the cell. And when we talk about the development of life into multi-cellular forms and movement onto land, the fundamental process there is the development of oxygen through photosynthesis and aerobic respiration.

Origins of life lecture

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